1. Field of the Invention
The present invention relates to optoelectronics fabrication. Particularly, the present invention relates to a method for fabricating a thin film light emitting device, having a metal cathode with anti-oxidization function.
2. Description of the Related Art
The current optoelectronics technology has been successfully developed. The light emitting diode (LED) is one usual product of the optoelectronical devices. The light emitting diode can emit light when is applied with a bias. Thereby, the light emitting diode, associating with circuit design, can be incorporated in a display device, or providing as a light source. The light emitting diode has very wide applications.
In general, LED can be characterized into two types. One type is metal semiconductor LED and the other type is organic LED. The organic LED can further divided into two types. One is small molecular weight LED and the other one is polymer LED. The basic operations of these LED is that when a voltage of 2 volts to 10 volts is applied on the two electrodes, the organic LED then emitting light.
The mechanism for the organic LED to emit light is due to recombination of electrons and holes in the organic material. As electrons and holes recombine, energy is released in form of light. Usually, when a voltage is applied on the organic light emitting material, a ground state of the organic light emitting material transits to an excited state. At this condition, the organic light emitting material is at an unstable state and will fall back to the ground state from the excited state. At this moment, the electrons and the holes recombine and photons radiate out.
The brightness of the organic LED has strong dependency one the transmitted energy by the electrons and the holes. When the number of pairs of the electrons and holes for recombination is larger, the light brightness is higher accordingly. Theoretically, the organic light emitting material is a kind of organic semiconductor, the transit energy of the electron-hole pair is affected by the energy band of any contact material. For the cathode, electrons are transmitted from the metal cathode to the organic light emitting material. If the energy band between the cathode material and the organic light emitting material is larger, electrons are more difficult to transit to the organic light emitting material. Therefore, the metal material having smaller working function, such as Mg, Li, or Ca, is useful to reduce the energy band gap between the metal cathode and the organic light emitting material. As a result, the metal with small working function is helpful for transiting electrons to the organic light emitting material. This also means that the light emitting device has higher emitting efficiency.
FIG. 1 is a schematic drawing, illustrating a typical structure for an LED. In FIG. 1, there is a transparent substrate 50. A transparent anode layer 52, such as indium tin oxide (ITO), is formed on the substrate 50 thereon. An LED material layer 54 is formed on the transparent anode layer 52. A metal cathode layer 56 is formed on the LED material layer 54. When a voltage is applied on the LED material layer 54, the lights 58 are emitted through the transparent substrate.
The metal cathode 56 usually includes a metal material with low working function, such as Ma, Li, or Ca, which metal materials basically are categorized as active metal. The active material is very easy to react with water or oxygen, resulting in degrading of performance of the metal cathode. Currently, the metal cathode of LED is usually made of active metal with low working function. Due to the active properties of the metal, the lifetime of production strongly depends on the amount of water or oxygen contained by the device. If the water or oxygen contained by the device is high, water and oxygen do react with the active metal cathode 56, causing the failure of the device. In order to ensure that the metal cathode is not eroded by water and oxygen, it needs a reliable package manner to package the device, so as to prevent the device from being eroded by water and oxygen.
Basically, the structure of device after packaging is shown in FIG. 2. After the metal cathode is accomplished, a sealant layer 60 is formed to cover a transparent anode 52, the LED material layer 54, and the metal cathode layer 56. Then, a cap layer 62 covers a portion of the sealant 60 above the metal cathode 56.
In order to solve the erosion of water and oxygen on metal cathode layer 56, the prior technology may add one additional material layer to reduce the reaction of the metal cathode with water and oxygen. However, the efficiency is still poor, or the method is more complicate.